Docker doesn't work anything remotely like a hypervisor. It doesn't provide the much greater assurance of security, scalability, isolation, resource metering, accounting, or flexibility that a hypervisor does. Docker is a security disaster and it only runs Linux. Xen/Qubes runs Windows, BSDs, or any other OS. Docker seems "easy" but with many subtle costs that come later at scale. You can't live migrate a Docker container from one host to another, where you usually can with Xen using shared storage. There are many other gotchas in the lifecycle of Docker containers that are eliminated or mitigated by using hypervisor guests instead.
flatpak and snap are basically filesystem overlays. They're gross, poorly-managed, incompatible duplications of package management.